1. Field of the Invention
The present invention relates to a neck adjustment mechanism for a string instrument, and more particularly to a guitar neck adjustment mechanism which maintains the pitch and the intonation qualities during adjustment of the neck.
2. Description of the Related Art
As is well known in the art, the primary quality attributes of guitars are tone (i.e., the audible nature of the instrument including volume, brightness, evenness, note separation, etc.), playability (i.e., the responsiveness of the instrument to the player's technique) and durability (i.e., the ability of the instrument to deliver tone and playability over years and decades).
With respect to playability, a critical aspect of playability is string action. Since each string on a steel string guitar is stretched to nearly thirty pounds of tension, the force required to fret the string is not insignificant. If the action is too high, playing is difficult, unpleasant and, in extreme cases, can cause repetitive stress injury. If the action is too low, the strings will “buzz” on the frets or may actually rest upon on the frets, making the instrument generally unplayable. The acceptable range for action is quite small—perhaps 0.1 of an inch or so. In view of this, guitars must be built very precisely with respect to neck angle and must maintain that critical geometry throughout time under the stress of nearly 180 pounds of string tension. It is to be appreciated that a rigid guitar structure generally tends to be excessively heavy and may compromise tone.
A lighter guitar structure tends to sound better with the risk that the neck may eventually pull up over time, altering the action of the strings to the point where the neck must eventually be reset, typically entailing a costly repair of many hundreds of dollars. Accordingly, the tone, the playability and the durability are fundamentally in combat with one another.
Since guitars are made mostly of wood, it is to be appreciated that the wood tends to move over time, not only under string tension but also in response to day to day humidity changes. A guitar with comfortably low action in Houston, Tex. may shrink enough, if flown to Minneapolis, Minn. for example during the winter, to be generally unplayable. The builder of the guitar must anticipate that the guitar may spend some time in low humidity so the stringed instrument must built with sufficiently action high to remain playable under all foreseeable circumstances. Unfortunately, generally the action will be sub-optimized when the humidity is higher.
As a result of the above, guitars normally tend to have an action that is higher than desirable to allow for the possibility that the stringed instrument will eventually experience a low humidity environment. As string tension gradually deforms the wood structures over time, the action is likely to increase and progressively get worse. Modification of the action of the stringed instrument, by the musician/owner/technician/repair person, is typically hampered by the fact that many guitars have fixed necks which prevent any relatively easy adjustment of the string action.
One approach of attempting to modify the action of a guitar, with a fixed neck, is to unstring the guitar and then remove and shave the saddle. Since the height of the saddle is typically not very high, the saddle must be significantly shaved in order to have any real effect on the string action, and it is to be appreciated that this may only temporarily solve the problem. Moreover, a short saddle tends to reduce the leverage that the strings have to vibrate the top surface of the guitar body so both the tone and the volume of the guitar are generally compromised to some extend.
More often, the musician/owner/technician/repair person will attempt to adjust a truss rod. A truss rod generally consist of a threaded rod, with nuts located on either end thereof, which extends parallel to another rod or bar. By rotating the threaded rod in one direction or the other, the truss rod eventually begins to bend thereby causing the neck and associated fret board to also correspondingly bend. It is to be appreciated that using the truss rod to compensate for more than a few thousands of relief is generally a bad option because such adjustment frequently results in a broken truss rod and this typically leads to the guitar eventually being discarded by the owner.
Some builders have incorporated various mechanisms which adjust the neck/body geometry. Stauffer/Martin and Howe Orme are examples of renowned builders who, more than a century ago, employed adjustable neck systems. A number of builders today employ neck-to-body joins that can be adjusted in one manner or another. However, only a small fraction of all guitars have such neck adjustment systems.
The most common approach is to enable the headstock end of the neck to “tilt” slightly in relation to the body, e.g., pivoting where the neck heel contacts the body and the pivoting is controlled by a screw extending through the neck heel into the body well below the pivot point(s). Rotation of the screw in a first direction pulls the heel closer to the body and effectively pulls the neck back, reducing the distance from the strings to the fret board and lowering the string action—U.S. Pat. No. 7,157,634, for example. Because the pivot point is well below the plane of the strings, such tilting also increases the distance between the nut and the saddle. Since the strings are already under approximately 180 pounds of tension, considerable force must be applied by the adjustment mechanism so prudence may require the guitar to be unstrung before an adjustment is attempted. In any event, any stretching or relaxing the strings will change the pitch of the strings, thereby requiring the player to retune the guitar following adjustment thereof. It is to be appreciated that a significant adjustment may change the distance between the nut and the saddle enough that the new effective scale length no longer matches the layout of the frets and the instrument may sound out of tune.
Another approach is to raise and lower the entire neck with respect to the guitar body using, for instance, a sliding mortise and tenon joint. Such a system is described in U.S. Pat. No. 7,557,281, although other “elevator” systems are available and known in the art. Such elevator system typically also stretch or relax the strings, for a given change in action, but typically less than a tilt system discussed above. However, even if the direction of travel is very close to being precisely perpendicular to the string plane, some stretching or relaxing of the strings will typically occur as a matter of geometry, which changes the pitch of the strings.
Moreover, both approaches generally require a wrench, key or some other tool to operate the adjustment mechanism and may also require some combination of unstringing, adjustment, restringing and retuning. As such, these mechanisms require the musician/owner/technician person to first locate the required tool(s) and then perform the desired adjustment operation. To the extent that either mechanism is well-built and adjustments are properly made, either system may enable the player to maintain the action of the instrument within an acceptable range. However, such maintenance is fundamentally an off-line, technical process to be executed from time to time as the seasons change or the player travels from one climate to another.